1. Field of the Invention
The present invention relates to a harmonic oscillator for high frequency in, for example, microwave or millimeter-wave band, using a transmission line resonator, and more particularly a harmonic oscillator suitable for obtaining fourth harmonic having a frequency four times as high as that of a fundamental wave or second harmonic having a frequency two times as high as that of the fundamental wave, among harmonics related to the fundamental wave of an oscillation frequency.
2. Description of the Related Arts
A high-frequency oscillator is employed in an optical communication system or its peripheral equipment, for example, and high performance and low cost are required. As such a high-frequency oscillator, the present inventors have already proposed a harmonic oscillator using a transmission line resonator in each of US 2003/0090332 A1 and US 2005/0174182 A1. These harmonic oscillators are adapted to obtain even-order harmonics such as second-order harmonic (i.e. second harmonic) or four-order harmonic (i.e. fourth harmonic) related to a fundamental frequency (i.e. fundamental wave) of resonance in the resonator, with a simple configuration.
FIG. 1A is a plan view showing a conventional harmonic oscillator generating second harmonic and FIG. 1B is a view showing voltage displacement distributions of standing waves in the harmonic oscillator shown in FIG. 1A. Similarly, FIG. 2A is a plan view showing a conventional harmonic oscillator generating fourth harmonic and FIG. 2B is a view showing voltage displacement distributions of standing waves in the harmonic oscillator shown in FIG. 2A.
Each of the second harmonic oscillator and fourth harmonic oscillator consists of transmission line resonator 1 which comprises a transmission line having its both ends as electrical open ends, and active elements 2 for oscillation. Active elements 2 act as negative resistances. The transmission line resonator 1 is provided as microstrip line resonator 1A, for example. Microstrip line resonator 1A has a signal line on one main surface of substrate 3 made of a dielectric and a ground conductor on the other surface of substrate 3. The ground conductor is provided over almost entire surface of the other main surface. The length of the signal line, which is an electrical length of the signal line including effects of the dielectric coefficient of the substrate, is λ0/2 where λ0 is a wavelength corresponding to an oscillation frequency (fundamental wave f0) and the signal line is provided in a linear form with its both ends as electrical open ends. Active elements 2 are connected to either ends of microstrip line resonator 1A, i.e. either ends of the signal line, each with capacitor 4 intervened. Capacitors 4 are intended to make coupling between resonator 1A and active elements 2 to be loose coupling, thereby enhancing independence of microstrip line resonator 1A.
In each of these oscillators, because microstrip line resonator 1A has its both ends as electrical open ends, two oscillating systems can be obtained in which active elements 2 for oscillation at both ends oscillate in opposite phase to each other with common microstrip line resonator 1A. In microstrip line resonator 1A, as illustrated in FIGS. 1B and 2B, fundamental wave f0 occurs as a standing wave having maximum voltage displacement portions in opposite phase to each other at both ends of resonator 1A and a minimum voltage displacement portion (i.e. zero potential point) at a midpoint portion of resonator 1. In addition, even-order and odd-order harmonics are generated as standing waves based on fundamental wave f0.
In this case, odd-order harmonics have voltage displacement distributions which are odd-symmetric with respect to the midpoint portion of microstrip line resonator 1A as a zero potential point and both ends are maximum voltage displacement portions in opposite phase to each other, as with the fundamental wave. On the other hand, even-order harmonics have voltage displacement distributions which are symmetric with respect to the midpoint portion of microstrip line resonator 1A as maximum voltage displacement portion and both ends of resonator 1A are maximum voltage displacement portions in phase or opposite phase in relation to the midpoint portion. In the figures, reference character f0 denotes fundamental wave, reference character 2f0 denotes second harmonic, reference character 3f0 denotes third harmonic, and reference character 4f0 denotes fourth harmonic.
At the midpoint portion of microstrip line resonator 1A, microstrip line stab 5 having a length of about λ0/4 where λ0 is a wavelength corresponding to fundamental wave f0 is provided so that the midpoint of resonator 1A is an electrical short-circuit end for the fundamental wave f0 component. By means of stab 5, the midpoint portion which is a minimum voltage displacement portion of microstrip line resonator 1A is forcefully brought to be a zero potential point for fundamental wave f0 so as to further ensure the symmetrical voltage displacement distribution.
Further, in the second harmonic oscillator shown in FIG. 1A, output line 6 is connected to the midpoint portion of microstrip line resonator 1A with capacitor 4 for loose coupling intervened therebetween. In this case, because the midpoint portion of microstrip line resonator 1A is a zero potential point for the fundamental wave f0 component, the fundamental wave component does not basically appear on output line 6. Similarly, because the midpoint portion is a zero potential point also for odd-order harmonics, the odd-order harmonic components do not appear on output line 6, as well. On the contrary, in the case of even-order harmonics including second harmonic, the midpoint portion of microstrip line resonator 1A is a maximum voltage displacement portion as described above. Therefore, when output line 6 is connected at the midpoint portion, the even-order harmonics are outputted from output line 6. In this case, because output levels (i.e. amplification levels) of harmonic components decrease as the order of harmonics increases, second harmonic is eventually outputted as a main component from output line 6.
In the fourth harmonic oscillator shown in FIG. 2A, one ends of output lines 6a, 6b are connected to points at a distance λ0/8 from either ends of microstrip line resonator 1A while the other ends of output lines 6a, 6b are connected in common. In this case, fundamental wave f0 and odd-order harmonics having the order of three or more have voltages with opposite signs to each other at two points at a distance λ0/8 from either ends, as shown in FIG. 2B with circles. Therefore, the fundamental wave component and odd-order harmonics components are not obtained from output lines 6 connected in common, because these voltages cancel with each other. On the contrary, in the case of even-order harmonics, for example second harmonic and sixth harmonic (not shown), points at a distance λ0/8 from either ends, i.e. points dividing a length between the midpoint and the end points of the transmission line into two equal parts are zero potential points. Therefore, second harmonic and sixth harmonic are not outputted. Because the points at a distance λ0/8 from either ends are maximum voltage displacement portions in phase to each other for fourth harmonic, fourth harmonic is outputted from output line 6. In this case, although 4n-th harmonics such as eighth harmonic or twelfth harmonic are also outputted from output line 6, these harmonics have the large orders and eventually fourth harmonic is outputted as a main component from output line 6, as described above.
It is also possible that output waveform of active element 2 for oscillation is distorted to relatively increase level of a harmonic related to fundamental wave f0 so that second harmonic 2f0 or fourth harmonic 4f0 can be more easily extracted. In addition, although microstrip line resonator 1A takes a linear form, it may take a meandering curve form or even an annular form. Further, the transmission line resonator may be formed not only as a microstrip line resonator, but also as a slot line resonator, for example.
However, in the harmonic oscillators having the above describe configuration amplification level of a harmonic decreases as the order of the harmonic increases, with the largest amplification level in fundamental wave f0. This is due to that active elements 2 for oscillation are connected to both ends of microstrip line resonator 1A which are maximum voltage displacement portions. Therefore, in both second harmonic and fourth harmonic oscillators, it is required to suppress amplification levels of the fundamental wave and harmonics which have lower orders and larger amplification level than that of second harmonic and fourth harmonic, respectively, among other things.
In the case of the second harmonic oscillator, because only one output line 6 is connected only at the midpoint portion of microstrip line resonator 1A, positioning accuracy of the connection position of the output line can be relatively high and therefore it is easy to suppress fundamental wave f0 having the largest output level particularly. However, in the case of the fourth harmonic oscillator, because output lines 6 are connected at two points at a distance λ0/8 from either ends of microstrip line resonator 1A, there is a problem that fundamental wave f0, second harmonic 2f0, and third harmonic 3f0, which have larger amplification levels than that of fourth harmonic 4f0, can not be sufficiently suppressed due to mainly imbalance in the circuit configuration, for example.
Further, as described above, active elements 2 for oscillation are connected at either ends of the transmission line which are maximum voltage displacement portions of microstrip line resonator 1A. At the maximum voltage displacement portions, impedance is theoretically infinite and input impedance is substantially large indeed. Therefore, impedance matching with active elements 2 is difficult, which results in complicated design of the harmonic resonator. Because of the high input impedance, phase noise characteristics in active elements 2 for oscillation are also degraded.